Welcome back ladies and gentlemen of the internet. When merging two families together, it's important to maintain tact, meaning that no child feels they're unappreciated or unequal to the others. It means that each parent does their part as adults to make everyone feel included, and to form bonds with their new step-children. Not everyone has this sensitivity or perspective though.
In this Reddit thread, the original poster (OP) is a frustrated father. He's been married to his wife for years, and they're still trying to come to terms with what it means to be in their family. He has one son from another partner, and she has four children from another partner as well. Since she has the dominant children, it's easy to isolate her step-son, which is what she tries to do. Even when taking a family photo, she screams at her step-son to get out of the shot, and that she only wants it to be her family. I would be livid too if I were the father.